Optical waveguides – Directional optical modulation within an optical waveguide – Magneto-optic
Reexamination Certificate
2001-10-26
2003-04-01
Healy, Brian (Department: 2874)
Optical waveguides
Directional optical modulation within an optical waveguide
Magneto-optic
C385S039000, C385S042000, C385S045000, C359S280000, C359S281000, C359S282000, C359S283000, C343S741000, C343S756000
Reexamination Certificate
active
06542647
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to an optical signal transmission system and an improved structure of a magneto-optical modulator for use in the same which utilizes the Faraday effect designed to modulate an optical beam up to a higher frequency.
2. Background Art
Most of external optical modulators employed in typical optical signal transmission systems utilize the electro-optical effect (i.e., Pockel's effect). Particularly, most of optical signal transmission systems for use in optical communication employ optical waveguide modulators utilize the electro-optical effect of an LiNb03 crystal (e.g., Nishihara et al., “Optical Integrated Circuit” published by Ohm Company, pp. 298-304 (1985)). The optical modulators using the electro-optical crystal, however, experience dc drifts (e.g., J. Appl. Phys. Vol. 76, No. 3, pp. 1405-1408 (1994)) or optical damage and has a difficulty in maintaining the reliability for long use. Alleviating such a characteristic deterioration involves an increase in manufacturing cost.
In recent years, there have been proposed optical signal transmission systems which receive an electric wave through an antenna and apply it to an electro-optics modulator in the form of a high-frequency signal (e.g., Japanese Patent First Publication Nos. 4-172261 and 10-186189).
Magneto-optical modulators using the magneto-optical effect have been studied for a number of years (e.g., Appl. Phys. Lett. Vol. 21 No. 8, pp. 394-396 (1972)), but they are still not in practical use because their response frequencies are lower than those of electro-optics modulators and thus being researched for use as optical magnetic field sensors or current sensors (e.g., J. Appl. Phys. Vol. 53 No. 11, pp. 8263-8265 (1982) and National Technical Report, Vol. 38 No. 2, pp. 127-133 (1992)).
Japanese Patent First Publication No. 7-199137 teaches the use of an magneto-optical modulator as a polarization modulator in an optical signal transmission system. The response rate of the magneto-optical modulator is, however, as low as several tens kHz. U.S. Pat. No. 6,141,140 teaches the use of an optical isolator as a magneto-optical modulator, but its response rate is also low. This is because typical optical isolators are usually covered with a metal member or employ metal magnets for applying a dc magnetic field thereto, so that application of a high-frequency field will result in generation of the eddy current, which makes it difficult to apply a high-frequency field at several tens kHz or more from the outside of the optical isolator. The optical isolator is in practice employed as an optical modulator and has the disadvantage that changing the degree of the magneto-optical effect (i.e., the Faraday effect) using an external magnetic field causes the light to be returned to an unwanted direction (i.e., toward a light source).
In recent years, magneto-optical modulators designed to measure the current in a semiconductor electric substrate in which a dc bias field is applied to a magneto-optical crystal film are also researched (e.g., Appl. Phys Lett. Vol. 68 No. 25, pp. 3546-3548 (1996) and Extended Abstract (61th Annual Meeting, 2000), The Japan Society of Applied Physics, University of Tokyo, 2000, 4p-Q-4).
Most of typical optical signal transmission systems are designed to modulate the driving current to be applied to a semiconductor laser at higher frequencies or utilize an optical waveguide modulator exhibiting the electro-optical effect (i.e., the Pockel's effect). Directly modulating the driving current applied to the semiconductor laser does not require a special modulator, thus providing the advantage that the optical signal transmission system will be simple in structure. It is, however, difficult to modulate the light emitted from the semiconductor laser at frequencies higher than several GHz. Additionally, actuating a driver of the semiconductor laser at higher frequencies may result in a failure in desired modulating operation or difficulty in transmitting an output far away because of laser chirp due to the high-speed modulation.
Further, in an optical signal transmission network consisting of a great number of optical fibers, an optical signal usually contains noises arising from multi-reflection from optical parts installed in each transmission line. In order to avoid this problem, a light source such as an LED having a wide emission spectrum is employed occasionally. The frequency band in which the LED can be energized is, however, on the order of 100 MHz (see Hiroo Yonetsu, “Optical Communication Device Engineering” published by Kogaku Shoin, pp. 135-141 (1991)) thus requiring a special optical modulator for modulating an output of the LED at frequencies higher then 100 HHz.
Optical waveguide modulators utilizes the electro-optical effect. In this case, the Pockel's effect is capable of high-speed modulation of a laser beam or light produced by an LED and does not encounter the problem of chirping, but faces, as described above, the problems of dc drifts and optical damage. Alleviating these involves an increase in manufacturing cost. In optical signal transmission systems in which an electric signal received by an antenna is used to modulate an optical beam (i.e., optical carrier wave), a modulator is usually installed in the open air and thus will have the problems of the dc drifts and optical damage. Further, most of optical waveguide modulators using the Pockel's effect are designed for a single mode. It is usually difficult to produce a waveguide having a greater core diameter of several tens of &mgr;m to several hundreds of &mgr;m. The optical waveguide modulators, thus, encounter the problem in that it is difficult to modulate at high speeds an output of a LED which requires an optical fiber having a great core diameter for transmitting a sufficient quantity of light, an output of an optical amplifier which is increased greatly in power, or an output of a fiber laser (greater in core diameter than several tens of &mgr;m.
Magneto-optical modulators utilizing the Faraday effect are also being researched which apply a dc bias field in parallel to a magneto-optical crystal film installed in a transmission line fabricated on a semiconductor substrate or a microstrip line and monitor the waveform of current flowing through the line. The structure which monitors the waveform of current flowing through the line on the semiconductor substrate (e.g., Appl. Phys. Lett. Vol. 68 No. 25, pp. 3546-3548 (1996)), however, faces the problem in that the unadjustment of impedance of the transmission line on the substrate causes the wave ringing. The structure does not use an optical fibers as an optical transmission line and is unsuitable for optical signal transmission systems. The other structure which measures the waveform of current flowing through the microstrip line (e.g., Extended Abstract (61th Annual Meeting, 2000), The Japan Society of Applied Physics, University of Tokyo, 2000, 4p-Q-4) has an analyzer disposed behind an optical fiber connected to an output of a magneto-optical element and poses the problem in that increasing the length of the optical fiber will cause a linear polarized light to experience random polarization in the optical fiber, thus resulting in a difficulty in modulating the intensity an output of the analyzer. Further, the above magneto-optical modulators are designed to apply the dc bias field to the magneto-optical crystal film in the same direction as that of application of a high-frequency field. The application of the dc bias field great enough to change the magneto-optical crystal film into a monodomain structure will cause the magneto-optical crystal film to be saturated magnetically, thus resulting in an decrease in magnitude of a modulated output signal or failure in outputting a modulated signal.
SUMMARY OF THE INVENTION
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to pro
Ishiko Daisuke
Ishizuka Satoshi
Itoh Nobuki
Minemoto Hisashi
Healy Brian
Matsushita Electric - Industrial Co., Ltd.
Parkhurst & Wendel L.L.P.
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